A vehicle-mountable automatic transmission is constructed by combining a torque converter and a speed change gear mechanism, and configured to selectively engage one or more of a plurality of friction elements such as clutches and brakes depending on a vehicle driving state, to change an operating state of the speed change gear mechanism, i.e., a power (driving force) transmission path, thereby automatically performing a speed change to establish a given speed stage. In this type of automatic transmission, a speed change between given two speed stages is performed by, among the plurality of friction elements, disengaging a given disengagement-side friction element and concurrently engaging other engagement-side friction element, in some cases.
During the above speed change, it is necessary to perform an operation of disengaging the disengagement-side friction element and an operation of engaging the engagement-side friction element, at an adequate relative timing. If the engagement-side friction element engaging operation goes too much ahead of the disengagement-side friction element disengaging operation, a state occurs transiently in which both of the friction elements are engaged. Consequently, the automatic transmission is placed somewhat in an interlocked state, which gives rise to a phenomenon, so-called “pull-in of output torque”. On the other hand, if the engagement-side friction element engaging operation lags too much behind the disengagement-side friction element disengaging operation, a state occurs transiently in which both of the friction elements are disengaged. Consequently, the automatic transmission is placed somewhat in a neutral state, which gives rise to a phenomenon, so-called “blow-up of engine speed”. These phenomena give a passenger uncomfortable feeling.
Therefore, it is desirable that an automatic transmission speed change operation is completed as quickly as possible, while avoiding the above phenomena. For this purpose, when an engagement operating pressure is supplied to an engagement-side friction element, a pre-charge can be performed to allow an oil passage extending from an oil pressure control valve for controlling the supply, to the engagement-side friction element, and an oil pressure chamber of the engagement-side friction element to be quickly filled with operating oil, as described, for example, in the following Patent Document 1.
After the pre-charge and then after an elapse of a holding period in which the operating pressure is held approximately constant for a given period of time, an engagement control is performed which is configured to raise the operating pressure again to fully engage the engagement-side friction element. In this case, an operation of the oil pressure control valve to supply the operating pressure is controlled by a control unit, to allow an operation of disengaging the disengagement-side friction element and an operation of engaging the engagement-side friction element to be performed at an adequate relative timing, as mentioned above.
In such an oil pressure control during a speed change, when the relative timing between the disengagement-side friction element disengaging operation and the engagement-side friction element engaging operation is not adequate, it is common practice to adjust the timing by controlling the engagement-side friction element engaging operation. However, it is not easy to accurately control the engagement-side friction element engaging operation to allow the timing to become adequate, while avoiding a situation where a speed change period becomes longer. Particularly, when a control operation of raising the operating pressure to fully engage the engagement-side friction element is started just after completion of the pre-charge, a shock is likely to occur due to rapid engagement of the engagement-side friction element.